The present invention relates to a method of inspecting an optical ununiformity (defect) of a transparent material such as a glass substrate serving as a transparent substrate for a photo mask or a transparent substrate for an information recording medium and an apparatus therefore. More particularly, the invention relates to a method of inspecting an ununiformity of a transparent material and an apparatus therefore, whereby the ununiformity of the transparent material can be detected at a high sensibility and a high speed by using characteristics of a total reflection on the surface of the transparent material and its apparatus.
In a manufacturing process of a semiconductor integrated circuit, a photo mask, or the like, a photolithography method is used to form a fine pattern. For example, when the semiconductor integrated circuit is manufactured, a pattern is transferred onto a transparent substrate which was mirror-finished by polishing at a high precision by using a photo mask whose pattern was formed by a transparent film (for example, a chromium film). As a method of inspecting the photo mask which can be said as an original board of the pattern, as shown in a surface state inspecting apparatus disclosed in Japanese Patent Application Laid-Open No. 58-162038 (1983), a method of converging light onto a fine region on a pattern surface and comparing a reflection output and transmission output from the pattern surface has been known.
In recent years, however, in association with a realization of high density of the pattern, in addition to the inspection for the pattern surface similar to the above method, a fine defect of the transparent substrate itself which was mirror-finished by polishing at a high precision is also regarded as a target of the defect detection. In the above-mentioned method, since the light is converged onto the fine region on the pattern surface, when an inspecting region extends over a wide range, it is necessary to scan the light by using some means, a long inspecting time is required in proportional to the area of the inspecting region, and a change in light quantity of the reflection light and transmission light for the pattern itself and transparent substrate is not large depending on the presence or absence of the defect, so that it is difficult to apply the method to the detection of the fine defect on the transparent substrate.
Also with regard to a transparent substrate for an information recording medium, from the viewpoints of the formation of an under layer and a magnetic layer having a good crystallinity which are formed on the surface of the transparent substrate, the low flotation of a magnetic head, and the like with the realization of high density recording, the transparent substrate having the surface polished at a high precision is required, so that the fine defect of the transparent substrate itself is also set to a target of the defect detection. However, existing defect inspecting method and apparatus do not necessarily satisfy a request of the defect detection.
According to the invention, therefore, in order to solve the above problems, it is an object to provide a method of inspecting an ununiformity of a transparent material, whereby an optical ununiformity of the transparent material can be certainly detected at a high precision and a high speed and an apparatus therefore.
Further, an object of the invention is to provide a method of inspecting an ununiformity of a transparent material, whereby a desired transparent material can be immediately extracted and an apparatus therefore.